Chytrid spreads from central points of initial infection into surrounding areas in a wavelike pattern over time—similar to how the Ebola or West Nile virus moves, the new findings show.

The results appeared in the March 25 issue of the journal PLoS Biology.

Environmental Trigger

If global warming was triggering chytrid outbreaks, amphibian declines would occur in multiple spots simultaneously, according to lead author Karen Lips, a zoologist at Southern Illinois University.

"The idea is that the fungus is a native thing that naturally occurs in these areas, and that some environmental trigger causes it to break out, going from some form that doesn't infect or kill frogs to something that does," said Lips, who has received funding from the National Geographic Society. (The National Geographic Society owns National Geographic News.)

But the team found no evidence of concurrent declines.

Lips cited an example of two geographically similar sites in Central America separated by about 30 miles (50 kilometers). Frogs were dying in one site but were safe in the other.

"If temperature change was causing this outbreak, then the temperature change 50 kilometers away at the same elevation should be about the same and cause the outbreak of disease there," Lips said.

Not everyone agrees with the new results. Biologists Camille Parmesan and Michael Singer of the University of Texas, Austin, called the team's analysis "questionable" in a response to the paper posted online on the PloS Biology Web site.

Parmesan claims that the team wrongly equated the date of the first observed amphibian decline in a region with the onset of chytrid infection.

"They're using the date of decline as a proxy, but they actually don't know when the fungus arrived," Parmesan told National Geographic News.

"The decline could be occurring because of anything—including climate change."

Parmesan and others suggest a third possibility: Global warming and the spread of natural disease are working together to reduce frog populations.

"The data's pretty crappy to be honest, but it's good enough to say that both the fungus and climate change are separately responsible for some population declines and some species extinctions," she said.